1. Field of the Invention
The present invention relates to a system for optically and electronically ascertaining the existence of precipitation and the nature of precipitation in ambient air from a remote location.
2. Description of the Prior Art
Automatic weather identification systems have previously been devised which employ laser scintillations and scattering. The purpose for such automatic weather identification is to provide for remote monitoring of weather without the necessity for human observation at the weather monitoring site. Such a technique allows precipitation conditions at unmanned airfields to be remotely monitored and reported to aircraft in flight.
The nature of precipitation can be ascertained by observing the effects of precipitation on visible and infrared wave transmissions through the atmosphere. A determination of the existence and nature of precipitation in the atmosphere can be performed by observing the increase in high frequency components caused by the fine interference patterns as falling particles of precipitation sweep past a horizontally oriented line apertured light detector. The advantage of using a horizontally oriented line apertured light detector is that it is sensitive only to the vertical motion of the falling precipitation particles. Rain induced scintillations contain substantial frequency components above one kilohertz. Snow induced frequencies are primarily below a few hundred hertz.
Conventional optical weather monitoring systems employ optical transmitters that have all relied upon sources of coherent light, typically laser beams, which require an extremely large distance of light transmission in order to obtain a statistically significant sampling of precipitation so as to ascertain the rate of precipitation. The necessary spatial separation between the receiver and the transmitter of such laser scintillation detection systems is on the order of fifty meters. This large spatial separation between the transmitter and receiver of a weather monitoring system is inconvenient for use in field operation, such as at airports, due to the large area which is required to effectuate automated monitoring. Furthermore, such conventional systems are subject to considerable contamination as a result of air turbulence which is likely to occur in ambient air over a distance of fifty meters. As a result, automated systems which have heretofore been implemented to replace human judgment have been utilized to only a very limited degree. Also, most other types of conventional automated weather monitoring systems can only measure total water content of precipitation in ambient air, and are unable to identify the type of precipitation, as between rain, snow or hail, without assistance from human observers.